The present disclosure relates to crystal-oscillator circuits having quartz crystal units, and more particularly to stabilization control of the oscillation frequencies.
Crystal-oscillator circuits having quartz crystal units are widely used as oscillator circuits which generate reference frequencies in electronic devices.
In recent years, the transmission data rates in electronic devices have been increased. Such electronic devices perform operations intermittently for the purpose of lowering the power consumptions. In addition, electronic devices are required to have not only a smaller size and a lower weight, but also high reliability and high accuracy. Given such a background, a need exists for a crystal-oscillator circuit to start in a short period of time, and to output a stable oscillation frequency with low power consumption; among others, an oscillation frequency is required to be highly stable against a change in the ambient temperature.
FIG. 6 is a diagram illustrating an example configuration of a conventional crystal-oscillator circuit. In the configuration of FIG. 6, a quartz crystal unit 65 and a MOS variable-capacitance element 60 and a fixed-capacitance element 62 in an oscillator circuit section 51 form an oscillation loop. A component 64 is an inverter, and a component 66 is a resistor. When a supply voltage 67 is applied, a regulated-voltage circuit 52 supplies a predetermined supply voltage to the oscillator circuit section 51, making an impact on the quartz crystal unit 65, thus oscillation starts. An oscillation signal is output from an output circuit 74. A voltage control circuit 73 applies a control voltage to the drain of the MOS variable-capacitance element 60 through a resistor 69 to change the MOS capacitance value. This causes the load capacitance CL of the oscillator circuit section 51 to be changed, and thus an output frequency of the crystal-oscillator circuit is adjusted to a preferable frequency f0.
A quartz crystal unit generally exhibits a temperature characteristic approximated by a cubic function. Accordingly, it is preferable that a function to compensate for the temperature characteristic of a quartz crystal unit be added to a crystal-oscillator circuit. For example, a technique is known in which a control voltage Vc having a temperature characteristic for compensating for the temperature characteristic of a quartz crystal unit is applied to a variable-capacitance element, which serves as a frequency adjustment element, to stabilize the temperature characteristic of the oscillation frequency. However, since generation of an ideal control voltage Vc is technically difficult, temperature compensation of the oscillation frequency is performed generally by generating a control voltage having a temperature characteristic of a quasi-cubic function in various ways. In the configuration of FIG. 6, a temperature compensation circuit 53 applies a control voltage for compensating for the temperature characteristic of the quartz crystal unit 65 to the gate of the MOS variable-capacitance element 60 through a resistor 68.
Next, a module configuration of the crystal-oscillator circuit will be described. The quartz crystal unit and an integrated circuit (IC) chip are packaged in a module by ceramic material. The quartz crystal unit and the IC chip are electrically connected through a gold wiring pattern and gold bumps in the module. The quartz crystal unit and the IC chip are thermally connected through the ceramic material, the gold wiring pattern, and the gold bumps.
In recent years, it is desired that a crystal-oscillator circuit have an excellent startup characteristic of frequency and excellent stability of frequency. In order to provide a high value-added crystal-oscillator circuit, improvement of these characteristics is essential. Japanese Patent No. 4167255 discloses an example of conventional crystal-oscillator circuit.